DESCRIPTION (Adapted from applicant's description): The Vitamin A and Zinc - Prevention of Pneumonia (VAZPOP) Study. Linking Vitamin A and Zinc Deficiencies, Immunity, Growth, and the Prevention of the Leading Cause of Childhood Death. The objective is to delineate how vitamin A and zinc supplementation interact in improving immunity, fostering growth, and preventing infection, in populations at risk for malnutrition and vitamin A and zinc deficiency. Malnutrition is involved in half of the global deaths in children less than 5. Acute respiratory infection (ARI), especially acute lower respiratory infection (ALRI) or pneumonia, is the leading cause of death in children. The investigators propose to conduct a randomized, placebo controlled, double blind, nutritionally stratified study of low dose vitamin A, 10 mg/day elemental zinc, both, or placebo in 2,400 children in Quito Ecuador. They will test the hypotheses that: a) low-dose vitamin A has paradoxically positive and negative effects on ALRI in underweight and well nourished children; b) zinc will protect against ALRI and diarrhea while boosting cell mediated immunity; c) growth will be fostered by zinc (and potentially by vitamin A) in deficient children; and d) misclassification of ALRI cases can mask the benefits or risks of vitamin A. The investigators will use state-of- the-art field techniques to assess body composition and growth, and utilize sophisticated techniques to assess vitamin A and zinc deficiency. In addition, they will use rigorous definitions of ALRI/pneumoma to avoid misclassification and ascertainment bias, which may have affected prior studies.